Techniques for making a master hologram or diffraction grating from which replicas--of metallized plastic--can be mass-produced are known. As used herein, the term "hologram" is meant to include diffraction gratings. Briefly and very generally, there exist two distinct types of holograms and two basic techniques for producing them for use as masters. For the purposes of this description, the term "hologram" will be used to refer to any suitable pattern (either surface relief, or volume), or replica thereof, regardless of whether light diffracted from it reconstructs an image of a recognizable object or some other desired light distribution at wavelengths within and outside of the visible spectrum. As a practical matter, a master is formed and used to make multiple copies. Again, there are several procedures for making copies.
"Surface relief" holograms are different from "volume" holograms in the structure that provides the holographic image. Surface holograms employ a pattern of lands and grooves at the surface to the physical substrate. On the other hand, a volume hologram functions by diffracting light from internal layers, or fringes of varying contrast or refractive index.
One type of hologram is a white-light viewable, reflecting transmission hologram or rainbow hologram. This type of hologram can be a true, three dimensional hologram, which reproduces the three dimensional image of an object and is typically produced using coherent laser light and photosensitive plates. The light pattern of the original object can be reconstructed from such a hologram by diffraction of a portion of illuminating light, provided that the illuminating light impinges upon the hologram at the correct incident angle and is comprised of the correct wavelength or wavelengths.
The other type of hologram involves the production of a diffraction grating that does not necessarily represent an image of an object and can be effectively produced by the of use of coherent laser light interference, microphotolithographic techniques, or direct-write techniques, such as electron beam etching.
In one useful form of the first of these two techniques, a master hologram (H1) is formed by conventional techniques. This hologram is then projected out into space by playing it back with a conjugate to its original reference beam. This projected, real image is then made to interfere with another reference beam, and forms a holographic image (H2) that can be viewed in white light--no laser is necessary. The H1 is shot on a suitable photosensitive emulsion (e.g., silver halide). If shot on silver halide, the H1 master will have no surface structure--just micro light/dark bands that diffract light. The H2 is shot on photo-resist so that there is a reproducible holographic surface relief pattern.
E-beam mastering essentially "carves" out a series of geometric grooves in a surface. This can provide only two-dimensional (but highly kinetic) imagery. There is no "object" wavefront being recorded--just what is called a plane grating.
Once a master surface relief hologram is formed by one of these or other procedures, the surface relief pattern is typically converted to a physically more rugged structure from which replicas can be made. A first step in a usual commercial process is to form a thin layer of silver that conforms to the surface relief pattern. The purpose of the silver layer is to render the surface relief pattern electrically conductive to facilitate later electrodeposition of nickel in the exact physical shape of the pattern. From this metal master hologram, a number of submasters are usually made. Each submaster replicates the master surface relief pattern and is used to make a large number of copy holograms.
There are at least three basic procedures being utilized to make copy holograms. In each, a surface relief pattern of a hologram master or submaster is employed to copy the hologram onto a desired carrier, typically plastic. Of course, copies of the surface relief holograms are actually castings or forms, which exhibit an inverse surface structure of the master or a submaster.
One procedure for making copies involves embossing a thermoplastic film (hard embossing) wherein the submaster is urged against thin plastic film under sufficient heat and pressure to transfer the surface relief pattern into a surface of the film. This technique is referenced by H. M. Smith in Principles of HOLOGRAPHY, 1969, at page 49, and by D. J. Pizzanelli in WO 93/00224.
A second procedure (curable casting) employs a curable resin layer which is contacted directly or indirectly with a master to create the relief pattern and cured, such as by UV radiation, to set the pattern in permanent form. One process using this technique employs a casting process wherein a liquid resin is trapped between the surface relief pattern of a submaster and a plastic film while actinic radiation or other curing technique, such as electron beam irradiation, hardens the resin. When the submaster and film are separated, a cast surface relief pattern remains attached to the plastic film. This process is described, for example, by K. Y. Kwon in DE 41 40 545 and by S. P. McGrew in U.S. Pat. No. 4,758,296. A variation of this technique is taught by Takeuchi, et al., in U.S. Pat. No. 4,856,857, wherein a nontacky polymerizable precoat is shaped under heat and pressure and cured while in contact with the submaster. A further process utilizing a curable resin again utilizes a curable resin layer on a substrate, but utilizes a resin that can be worked at ambient temperature and is cured after separation from the submaster.
A third procedure (thermal casting) is similar to both of the others above, but utilizes heat to soften a thermoplastic web, or the surface of one, to forming temperature before contact with the master or submaster. The surface embossed with the relief pattern in this manner is cooled to harden the plastic surface. This technique is shown, for example, by the Krug reference cited above, D. R. Benoit, et al., in U.S. Pat. No. 5,164,227 and Dainippon in Japanese published applications Nos. 50 46064 and 50 46065.
A next step, in both the casting and the embossing replication processes, is to coat the surface relief pattern with a thin layer of opaque, reflective material--usually, aluminum applied by vacuum deposition. The result for any of these processes is a hologram in which the recorded light pattern is reconstructed in white light diffracted in reflection from the coated surface relief pattern. The reflective hologram replica can then be attached by lamination or otherwise to a substrate, such as an aluminum can. There is no practical method known for directly forming the hologram onto a metallic surface without degrading the character of the metal substrate, or the optical fidelity of the holographic submaster.
Two recent patents have been identified which describe the formation of holograms directly onto metal substrates, such as aluminum; however, both are subject to practical limitations. One process cannot be utilized without heating and degrading the metal substrate. The other requires specific stamping equipment.
In U.S. Pat. No. 4,725,111, Weitzen, et al., describe a process that, unlike the art prior to them, e.g., thermal casting on metal, does not require softening the metal to the point of plasticity. They note that the yield strength of annealed aluminum decreases steeply with rising temperature. What they disclose is that there is an optimum operating temperature for embossing that is at a temperature which is at the high end of a steeply declining yield strength versus temperature curve. Thus, inherently, although the disclosed process is meant to limit degradation of yield strength, there is some degradation. And, unfortunately, surface deformation increases when heating lowers the yield strength. This can cause unacceptably "wrinkled" cans due to the embossing pressures used. The disclosure reveals no special techniques to aid in preparing the hologram master or an embossing shim that would assure high quality and durability. Simply, the skilled worker is told that conventional techniques (the same used to emboss softened plastic) are all that are needed. Applicants' experience shows that unheated aluminum substrates cannot be embossed with holograms using old techniques, and they have endeavored to identify the changes necessary to enable this result. There is a need to enable providing a means and method for embossing a large number of aluminum substrates without compromising quality of the microstructure or integrity the tool.
In U.S. Pat. No. 5,193,014, Takenouchi, et al., describe a coining procedure for imparting a hologram to an aluminum container. The disclosure describes the use of a thin metal plate mold having a concave-convex hologram or diffraction pattern. It is said to be preferred to make the plate as thin as possible to render it deformable. This is necessary in the preferred embodiment wherein the plate mold is supported on a cushioning member. Unfortunately, when the plate is deformable and constantly subjected to deformation, the pattern on the plate tends to become distorted due to stretching forces. Moreover, bending stresses are concentrated at the grooves in the plate, and permitting freedom to flex increases the likelihood of stress failure.
Again, in the description of Takenouchi, et al., as in the disclosure of Weitzen, et al., the preparation of the metal plate mold is not described in detail, other than to indicate that it is done as conventional. The use of a cushioning member and a flexible substrate indicates that an unhardened nickel bath is employed because a hardened surface would further increase the likelihood of stress fracture. The softness of conventional nickel plating and the cushioned backing will tend to cause diminishing results in terms of image clarity over shorter than desired number of embossing cycles.
The problems addressed for coining, either with the disclosed flat or spherically-rounded surfaces, do not translate well to cylindrical surfaces. The ability to use cylindrical shims and achieve high production rates and long production runs is beyond the scope of the Takenouchi, et al., teachings. Cylindrical embossing shims will not provide durability and will break, stretch or deform unless design considerations not addressed by that reference are taken into account. That patent illustrates a cylindrical shim in FIG. 5, but the disclosure does not provide any reference to it other than in the description of the drawings. The factors necessary to assure success using cylindrical shims are not addressed. Indeed, there is no disclosure as to how one can be made.
It would be desirable to have a means and a method for reliably embossing aluminum (and other metals and hard materials) which would be effective over long production runs to provide high quality transfers that do not materially reduce the strength of the aluminum substrate.